Constructors #2

In my last post I wrote about the problems with constructors. In this post I’ll describe the approach I’m currently considering using for my hobby language. But first I have to set things up a bit and describe some related language constructs.

Protocols

First of all, there are no classes but only protocols. You can think of protocols as classes without fields or, equivalently, as interfaces where methods can have implementations or, most accurately, as something very similar to traits. Here is an example:

In a class-based language Point2D would have had two instance variables, x and y. Since protocols don’t have fields this one has two virtual methods instead where the fields should be, x() and y(). It also has a to_string method which uses string interpolation, which I’ve stolen directly from the neptune language.

One way to create an instance of Point2D is to create a subprotocol that defines the methods x and y:

Even though protocols are like interfaces there is nothing to stop you from creating instances of them. And because they don’t have any fields, initialization is trivial.

Records

However, it obviously doesn’t work that you have to write a named subprotocol whenever you want a concrete point. Instead, there is a dual to protocols: records. Where a protocol is pure behavior and no state, a record is pure state and no behavior. In the case of Point2D the associated record could be

All a record can do is return the field value when the associated method is called. As with protocols, initializing a record cannot fail because it is only created after all the field values have been evaluated.

So now that we have behavior without state and state without behavior we just have to combine them. This is done using the “extended” method call syntax:

This syntax creates a record like the previous example, but make the record an instance of Point2D. Initialization is trivial, as with plain records, because all field values have been evaluated before the instance is created. This works in the simple case but unfortunately doesn’t solve the general problem. First of all, we’ve exposed the “fields” of Point2D. If we later want to change to a polar representation, every occurrence of the above syntax that binds x and y directly will give us broken points. Also, this does not take inheritance into account.

For this reason, the syntax above is not intended to be used in client code, only in Point2D‘s methods. Instead, Point2D should define factory methods for client code to call:

The static modifier means that the method is the equivalent of a smalltalk class method. x: and y: are python-style keywords with a more smalltalk-like syntax. The factory keyword does not have any effect until we get to subprotocols. In any case, with this definition you can now create a new point by writing

def p := new Point(x: 3, y: 4);

If, at some later point, you decide to switch to a polar representation you simply have to change the definition of new(x:, y:):

As long as people use the factory methods you are free to change the implementation of the protocol. Also, the protocol can do arbitrary pre- and postprocessing and the actual instantiation and initialization is atomic. All that’s left now is to account for subprotocols. Enter Point3D

This sort of works but it is error-prone, breaks encapsulation, and breaks down if we were to change Point2D to use polar coordinates. What we really want is for Point3D to call the new(x:, y:) method on Point2D. That’s where the factory keyword from before comes in. A factory method is one that allows you to specify a set of fields. Because new(x:, y:) is a factory method the instance created by this call

newthis() { x: i, y: j }

will not only have an x and y field but also any fields specified by the caller of new(x:, y:). This allows us to rewrite new(x:, y:, z:) as

Even though this requires nontrivial computation in a superprotocol there is no risk of creating improperly initialized objects. In terms of the construction steps described in the last post this allows initialization to proceed like this:

Arbitrary preprocessing in Point3D

Arbitrary preprocessing in Point2D

Object instantiation and full initialization

Arbitrary postprocessing in Point2D

Arbitrary postprocessing in Point3D

The fact that protocols don’t declare fields but that fields are provided upon construction also allows new kinds of specialization. For instance, we don’t actually have to change the implementation of Point2D to create an instance that uses polar coordinates:

In this case we choose to bypass the new(x:, y:) method and instead provide a method implementation of those fields instead of instance variables. This way we can share all methods in Point2D. This is also possible in other languages if all access to x and y takes place through accessors, but there you would get two unused fields in all instances.

Finally, let me just mention that any method, not just new, can be a factory; the syntax is just slightly different when you invoke it; for instance

def q := Point2D.at(x: 3, y: 4) { z: 5 };

It looks slightly different but the mechanism is exactly the same.

1 The meaning of new syntax, as in neptune, is not a special construct but an ordinary method call. The syntax new X() is exactly equivalent to the ordinary method call X.new().

5 Responses to Constructors #2

In Scala, as in Java, you have free access to the object being initialized during initialization and you can execute arbitrary code in context of that object. That has some unfortunate consequences. For instance, if a superclass constructor happens to invoke a method that has been overridden in a subclass then the method will be executed before the subclass has had a chance to initialize itself. This can cause some very subtle and unintuitive bugs, especially if there is no immediate error but you just silently propagate some inconsistent values. And of course if initialization code happens to throw an exception then you get a broken object. This model also makes it impossible to have real immutable fields and only a more complicated approximation, like finals in java, is possible.

The part worst part of scala constructors, though, is the way they are tied to pattern matching in case classes. There you get direct access to the object’s fields which ties code not only to a particular implementation of an object but to the way in which the object was constructed. That seems very un-object-oriented.

1. To Scala’s defense, a lot of design choices and limitations stem from the we-will-always-compile-to-JVM doctrine. This also includes that you should be knowing what you should not be doing when you call a virtual method from within a constructor.

2. In addition to those limitations, Scala’s virtual types are directly tied to constructors – if a class has a type member Foo, then (similar to generics), one has to make a choice for “type Foo”. Syntax like new Bar { type Foo = String; val x = 3; val y = 4 } is pretty elegant, consistent and straightforward and I find your comments regarding Scala constructors do not seem to cover this side of object construction (which looks somewhat similar to your own).

3. Sometimes I think un-object-oriented is a compliment, but more to the point, I agree that case classes are in conflict with data abstraction, that is why there are extractors now to provide a more fine-grained approach to pattern matching. In newer versions of Scala, a case class is translated to a static method call behind the scenes (case class Bar becomes object Bar with an “apply” method and a class Bar). The apply method plays the same role as the static factory methods.

4. (This could answer the question of the previous comment) If new Foo is the same as Foo.new, why provide syntax for it?

BTW, an answer for the question in the previous comment, if it is the same as in Scala, a constructor call new Foo(arg1,…,argN) becomes Foo.new(arg1,…,argN). For Scala case classes, a call Foo(arg1,…,argN) becomes Foo.apply(arg1,…,argN). I guess making it work with named parameters is not hard, as fixed-length records are really the same as tuples.

I do have a lot of opinions on Scala for someone who hasn’t really ever used it, don’t I. In this case, I didn’t actually know about the new Bar { … } mechanism. It does indeed look very similar and seems to avoid partial initialization problems (modulo the behavior of Bar’s constructors). However, as far as I understand the mechanism it only works for leaf classes, you have to use standard constructors to build deeper inheritance hierarchies. Is there a way to use this to express the example from the post where both Point2D and Point3D are free to switch their internal representation without requiring changes to each other or client code, and where partial initialization can’t occur?

(I’m curious — if you’re not a fan of object orientation do you know of an alternative solution to this problem in a different paradigm?)

Extractors do give the data abstraction I think is missing with case classes; I also like the pattern matching mechanism in F#. I’m not sure either solution gets it exactly right but on the other hand I can’t think of any way to improve them.

Even though new methods are no different from others and you might as well have called Point.at(x: 0, y: 0) I think the code becomes easier to read if people use the same method name more or less consistently. The special syntax is there to encourage that, and to make construction expressions stand out more. But in the language I’m working on syntax is cheap — the new expression is not built in, it’s just a convenience added by a library.

You’re right about the translation of new methods and, in the simple case, the implementation of keyword arguments is relatively straightforward. It’s a little more complicated than tuples since you have to be able to reorder the arguments. Calling f(a: 3, b: 4) and f(b: 4, a: 3) should have the same effect except for the evaluation order of the arguments.